This study aims to extend a recent body of research which has focused on younger siblings of children with autism spectrum disorders (SIBS-ASD) who are not only at elevated risk for autism spectrum disorders but also for general developmental lags in areas including joint attention (JA), language, and social communication. Early JA abilities are of particular developmental importance and have been shown to predict later outcome in terms of language abilities and quality of social relationships in typically developing children and in children with autism. Thus far, the large part of research on SIBS-ASD has concentrated on children under the age of three and it is still unclear whether said developmental differences in SIBS-ASD will remain over time, or if this group will "catch-up" to the trajectories of their typically developing peers. This distinction is crucial for parents and professionals who must discern whether this developmental lag requires special attention and intervention, in order to help these children be able to lead healthy and productive lives, in line with NICHD's mission. The proposed follow-up study will assess the language and social outcomes at 60 months (T6), in a group of SIBS-ASD (n=66) and a chronological age-matched group of younger siblings of typically developing children (n=37) whose developmental trajectories on JA are currently being followed from the age of 12-23 months (T1) to 29-43 months (T5) (at 5 time points). Language and social skills will be assessed at T6 follow-up using a multiple method approach. The main research questions are: (1) Are JA abilities measured at T1 predictive of later (T6) language and social skills?;(2) Does rate of JA acquisition from T1 to T5 (over 5 time points) predict outcome (i.e., language, social) at T6 follow-up?;and (3) Is language delay stable over time for the group of SIBS-ASD that demonstrate this deficit early on? This project represents a unique contribution to the literature as it will make it possible to evaluate the developmental functioning of SIBS-ASD over a longer developmental time span than has previously been studied. PUBLIC HEALTH RELEVANCE: This project will aid in determining the relative importance of early developmental lags in younger siblings of children with autism by examining whether one can use early joint attention skills to predict later language and social outcomes at age 5. The study hopes to clarify the degree of public health intervention that will be needed for diagnosis and treatment of these children as they approach school age. The findings of this proposed project will also be of foremost significance to parents and educators who are faced with the responsibility of selecting optimal educational contexts (i.e. classroom, school) for affected children.